This invention relates to magnetic memory elements of the type used to store and retrieve bits of data and to methods of fabricating the same.
The mass storage device most commonly used today in the computer industry is the disk drive. In a disk drive, data is stored in magnetic form on a rigid or flexible recording disk and is both stored and retrieved by means of one or more magnetic inductive transducer heads mounted on a mechanically translatable art. Data is stored in addressable sectors of concentric tracks, the addresses being recorded magnetically in appropriate locations. Because of the electromechanical nature of a disk drive, the speed with which data can be stored and retrieved is correspondingly limited. In spite of this limitation, and the additional limitations of any mechanical system involving friction and mass, the disk drive remains the most popular mass storage device in use today.
Prior to the development of relatively high capacity disk drive units, ferrite core memory arrays were widely used for mass storage in computer systems. A ferrite core memory system employs a plurality of planes of individual ferrite magnetic core elements and individual address and sense conductors used to isolate a single core element and either write or read information from that particular core element, with the aid of addressing gates. Although initially useful and an improvement over rotating magnetic drum memory systems, ferrite core memory systems suffer from several disadvantages. Firstly, a ferrite core memory system consumes a relatively large amount of electrical power during operation due to the relatively large drive current required for ferrite core magnetization and the relatively large voltage required for switching the magnetic flux found within the individual cores. Further, because operation of each core requires flux switching, substantial amounts of hysteretic heat are generated during use. In addition, the access speed of a ferrite core memory is relatively slow compared to present day disk drive units due to the relatively large drive current and large voltage noted above. Still further, a ferrite core memory system has an extremely small memory capacity when compared to a present day disk drive unit due to the relatively large physical size of each individual core element, when compared to the physical space on a memory disk required to store a given bit of information. Due to these disadvantages, ferrite core memory systems have been largely abandoned, except in certain computer system applications in which the high reliability of a ferrite core overcomes the above noted cost and performance limitations.
Attempts have been made to design practical magnetic memory systems devoid of the disadvantages of both ferrite core memory and disk drives. One such attempt is in the field of thin film memory systems, in which thin films of magnetic material are formed in an intersecting grid pattern using photolithographic techniques of the type employed in the fabrication of integrated circuits. In such thin film memory arrays, information is written into and read from the individual elemental sites to different combinations of electrical current usually pulsed through electrically conductive layers deposited on or sandwiched between the magnetic grid lines. Another attempt to provide a suitable mass storage array using integrated circuit photolithographic fabrication techniques employs crossing strips of magnetoresistive materials arranged in a grid pattern along with overlying electrically conductive sense lines which are insulated from the magnetoresistive elements. In such devices, the state of magnetization of a given site is measured by the value of standard electrical current passed through the sensing lines. Although much effort has been devoted to the attempted miniaturization of mass storage elements comparable in both physical size and performance to a magnetic disk drive, such attempts have not been successful.